1. Field of the Invention
The present invention relates to an optical disc apparatus in which an optical pickup can be maintained in a still condition (also called the on-track condition) over an optical disc.
2. Description of the Relevant Art
In optical disc apparatus, when an optical pickup traces an optical disc, rotated by a spindle motor, in a radial direction from inner to outer peripheral sides of the optical disc, an information signal recorded on the optical disc is read out by the optical pickup.
One of requirements of such optical disc apparatus is to repetitively reproduce an information signal recorded on a particular track of the optical disc, i.e., to maintain an optical pickup in a so-called still condition (also called on-track condition). According to the above still condition maintaining operation, when the optical disc is rotated by one track amount (one revolution) while the optical disc is being reproduced by the optical pickup, the optical pickup is moved by one track amount toward the inner peripheral direction along the radial direction of the optical disc, whereby the optical pickup is positioned over the same track on standby.
FIG. 1 of the accompanying drawings shows an example of a recording format of a magneto-optical disc that is one of optical discs now commercially available on the market. As shown in FIG. 1, a magneto-optical disc 1 has recording grooves that are spirally extended from an inner peripheral side to an outer peripheral side of a radial direction R. Although the recording grooves are spirally formed, one round of the recording groove is referred to as one track and the recording grooves are numbered by track numbers "nT" at every recording groove as 0T, 1T, . . . , 10T, 11T, . . . from the inner peripheral side in that order. Further, each track is divided into sectors S at an equal angular interval and the sectors S are sequentially numbered by sector numbers "nS" (e.g., 0S to 5S in the clockwise direction as shown in FIG. 1).
The arbitrary sector S comprises a header H and data D. On the header H, there is recorded in advance an address information having the aforesaid track number "nT" and sector number "nS" by an embossing treatment. In the magneto-optical disc 1 shown in the example of FIG. 1, address information from "10T0s" (sector 0 of 10'th track) to "11T5S" (sector 5 of 11'th track) are recorded on each respective header H. On the other hand, actual data information is recorded on the data D. Therefore, the information signal recorded on the magneto-optical disc 1 includes an address information and the data information.
The magneto-optical disc 1 thus arranged is rotated about a rotary shaft by a spindle motor (not shown) at a constant angular velocity (CAV), for example, in the direction shown by an arrow A in FIG. 1. The optical pickup 2 disposed in an opposing relation to the magneto-optical disc 1 is moved in the direction shown by an arrow R in FIG. 1 (in the radial direction of the magneto-optical disc 1), whereby contents of the header H and the data D which are the information signals within the sector S are read out from the magneto-optical disc 1.
In order to maintain the optical pickup 2 in the still condition over the magneto-optical disc 1 having the aforesaid recording format, the sector number "nS" within the header H is read out by the optical pickup 2. Then, when the sector number "nS" is changed from the sector number "5S" to the sector number "0S", there is generated a timing pulse and the optical pickup 2 is moved one track amount toward the inner peripheral side of the magneto-optical disc 1 in response to the timing pulse thus generated. This operation is called a one track jump operation. Alternatively, when the predetermined sector number of the sector number "nS", e.g., sector number "5S" is read out, there is generated a timing pulse and the optical pickup 2 is moved by one track amount toward the inner peripheral side of the magneto-optical disc 1 in response to the timing pulse.
In the magneto-optical disc 1 thus formatted as shown in the example of FIG. 1, one track corresponds physical to one disc revolution in a one-to-one relation (1:1). Also, the number of sectors within each track is equal. In other words, a sector structure is synchronized with one physical track.
Recently, there is proposed an optical disc which includes tracks, each of which has a (1/1.5) round to (1/2) round relationship, for example, relative to a physical track of one round (hereinafter referred to as a physical track), in other words, a track (hereinafter referred to as a logical track) in which a length of one logical track becomes less than the physical length of one round in the circumferential direction, i.e. less than the length of a physical track. The reason for this is that a recording density of the optical disc can be increased by selecting the physical length of the circumferential direction of the logical track to be substantially a constant length. Even when a track that has an asynchronous sector structure is formed, the number of sectors within each logical track is selected to be equal in order to facilitate the data processing or the like.
In the optical disc in which the logical tracks are formed as described above, it is frequently observed that the same sector number exists multiple times within one physical track. Therefore, when the optical disc is rotated by one physical track amount (one revolution) while the sector number is being read out, if the optical pickup is operated under the control of the conventional optical disc apparatus in which the optical pickup is maintained in the still condition (on-track condition) by moving the optical pickup by one physical track in the inner peripheral direction along the radial direction of the optical disc, then the optical pickup is sequentially moved toward the inner peripheral side of the optical disc so that the optical pickup cannot be maintained in the still condition.